This application is based upon and claims the benefit of priority of Japanese Patent Application No. H.11-329815 filed on Nov. 19, 1999, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a brake control device in which brake control can be automatically executed according to a demand based on vehicle behavior in addition to a demand based on a driver""s brake pedal operation.
2. Description of Related Art
Conventionally, a braking control for stabilizing vehicle behavior such as anti-skid control is well known. In the anti-skid control, when a brake pedal is depressed for braking, each vehicle wheel speed is controlled by adjusting braking hydraulic pressure of each wheel cylinder so that a slip ratio of each wheel may fall within a given range in order to obtain optimum braking force in each wheel.
To execute the anti-skid control, as shown in FIG. 15, a hydraulic circuit of a brake control device is provided with electromagnetic valves such as a pressure reduce control valve P1 and a pressure increase control valve P2 for reducing hydraulic pressure of the wheel cylinder or for holding hydraulic pressure of the wheel cylinder.
The pressure increase control valve P2 is a normally open valve (N/O valve) in which a fluid conduit is normally opened by a spring biasing force and the fluid conduit is closed when a solenoid is energized. The pressure reduce control valve P1 is a normally closed valve (N/C valve) in which a fluid conduit is normally closed by a spring biasing force and the fluid conduit is opened when a solenoid is energized. The pressure increase control valve P2 is operative to close a fluid conduit from a master cylinder P3 to a wheel cylinder P4 for holding the wheel cylinder hydraulic pressure when the pressure reducing valve P1 is closed and for reducing the wheel cylinder hydraulic pressure when the pressure reducing valve P1 is opened.
The hydraulic circuit is also provided with a check valve P5 (return valve) in parallel to the pressure increase control valve P2 (other check valves are omitted in the drawing). When wheel cylinder pressure is higher than master cylinder pressure, brake fluid returns from a master cylinder side to a wheel cylinder side. In a case that the vehicle is running on a high xcexc road such as asphalt road and a driver depresses suddenly and rapidly a brake pedal, the driver, who is a woman or elder person, is likely to return the brake pedal to reduce master cylinder pressure due to a reaction force of the brake pedal after the master cylinder establishes maximum pressure. When a brake pedal P6 is returned to reduce master cylinder pressure, the check valve P5 serves to return the brake fluid back from a side of the wheel cylinder P4 to a side of the master cylinder P3 so that wheel cylinder pressure may be reduced to lower the braking force, resulting in a longer braking distance.
In a special case where it is demanded that wheel cylinder pressure (braking force) is held even if the driver""s brake pedal depressing force is released due to a reaction force of the brake pedal or other reasons, the conventional hydraulic circuit having the returned valve mentioned above cannot work adequately.
Keeping a step with a recent tendency of developing a compact car having a light weight structure, it is demanded that the hydraulic circuit of the brake control device is more compact with a less number of brake components and parts but operative with better braking force holding function.
An object of the present invention is to provide a brake control device having a compact structure with a less number of brake components and parts for executing braking force holding control.
To achieve the object, the brake control device has brake pressure producing means (for example, master cylinder) for producing hydraulic pressure (for example, master cylinder pressure) according to a braking operation of a vehicle driver, wheel braking force generating means (for example, wheel cylinder) for generating braking force to a wheel, a fluid conduit connecting the brake pressure producing means and the wheel braking force generating means for allowing brake fluid flow between the brake pressure producing means and the wheel braking force generating means, a solenoid valve (for example, pressure increase control valve) disposed in the fluid conduit so that the fluid conduit may be provided on a side of the brake pressure producing means with a first hydraulic pressure and on a side of the braking force generating means with a second hydraulic pressure, an electric control unit, and sensor means including a pressure sensor for generating sensor signals representing vehicle states including a value of the first hydraulic pressure, all of which are input to the electric control unit.
The electric control unit has memory means for receiving the sensor signal of the pressure sensor and memorizing a peak value of the first hydraulic pressure, determining means for determining from the sensor signals a necessity of a braking force holding control for holding the second hydraulic pressure higher than the first hydraulic pressure when the brake pedal happens to be returned and generating a demand signal for executing the braking force holding control, calculating means for calculating a control amount based on a difference between the peak value of the first hydraulic pressure and an actual value of the first hydraulic pressure to be lowered when the brake pedal is returned, and driving means for driving the solenoid valve so as to restrict brake fluid flow from the braking force generating means to the brake pressure producing means in response to the control amount, when the demand signal is generated, wherein, while an increasing speed of the second hydraulic pressure is substantially equal to that of the first hydraulic pressure when the brake pedal is depressed, a reducing speed of the second hydraulic pressure is lower that that of the first hydraulic pressure during a given time when the brake pedal happens to be returned.
In the brake control device mentioned above, it is preferable to have an anti-skid control circuit provided with a hydraulic pump connected with the fluid conduit for the first hydraulic pressure and a pressure reduce control valve connected with the fluid conduit for the second hydraulic pressure in addition to the pressure increase control valve.
The braking force holding control is executed when the determining means generates the demand signal by determining based on the sensor signals that the driver depresses rapidly the brake pedal for sudden braking, for example, by determining that a change per time of the brake pedal movement exceeds a predetermined value.
Further, it is preferable that the braking force holding control is executed when the determining means generates the demand signal by determining that the vehicle stops on a sloping road due to the driver""s brake pedal operation or that an engine of the vehicle stops as an idle stop due to the driver""s brake pedal operation.
The sensor signals to be used for determining that the vehicle stops on the sloping road are, for example, signals from a sloping sensor for detecting an inclination angle of the vehicle stopping on the sloping road (the angle is more than a predetermined value), from a brake switch (ON) for detecting the driver""s brake pedal depression and from a speed sensor for detecting at least one of a vehicle speed and a wheel speed (zero speed).
Further, the sensor signals to be used for determining that the engine of the vehicle stops as an idle stop are, for example, signals from a sensor for detecting an engine stop (signal of fuel cut or signal of ignition stop), from a brake switch (ON) for detecting the driver""s brake pedal depression and from a speed sensor for detecting at least one of a vehicle speed and a wheel speed (zero speed).
Furthermore, the solenoid valve is, preferably, a two-position valve driven by the driving means so that current voltage applied thereto is controlled at a duty ratio in proportion to the control amount or a linear solenoid valve driven by the driving means so that a current amount applied thereto is controlled according to the control amount.